Clear popups on KEYCODE_ESCAPE as well as KEYCODE_BACK

On desktop devices, users expect that the Escape key will clear various
popup windows in the same way as the Back button will on mobile devices.
This adds support for Escape to places the Back button currently clears
ListPopupWindow, AutoCompleteTextView, PopupWindow, and ScrollView.

Bug: 174164194
Test: Open popups of each type on a device with a physical keyboard and
  press escape.
Change-Id: I32e65ce4317db7de043c8a050d87126b0f38acce